1. Field of the Invention
This invention relates to seals. More particularly, this invention relates to seals formed of a novel raw material. Further, this invention relates to seals to be used in the place of asbestos gaskets or packings, for example. This invention also relates to seals such as O rings, packings, etc. disposed in joints of pipes or containers laid in plants of the food industry, for example.
2. Prior Art
Asbestos is an inexpensive material excellent in resistance to heat, resistance to chemicals, resistance to wear, and tensile strength and, moreover, the fibers of asbestos are very soft. Thus, the asbestos is used extensively in the field of seals as gaskets or packings.
The asbestos dust drifting in the air, when inhaled in a large amount over a long period by a person, he is liable eventually to suffer from impaired health due to asbestosis or cancer of the lung. In recent years, the hazard has been raising a social issue. This environmental problem brings its influence to bear on seals. Recently, efforts have been directed to the development of seals made of a raw material taking the place of asbestos.
The constituent materials currently used for seals in the place of asbestos include sheets of such general-purpose rubbers as styrene-butadiene rubber, acrylonitrilebutadiene rubber, butadiene rubber, and chloroprene rubber, sheets of urethane rubber, sheets of polytetrafluoroethylene, polytetrafluoroethylene-coated asbestos sheet, carbon fiber knit, and Alamides fiber knit, for example.
The seals made of general-purpose synthetic rubber sheets, among other materials mentined above, have an advantage that low pressure suffices for squeezing, satisfactory field workability is exhibited, a high contracting-resiling property is displayed, and an electric insulating property is manifested. They are, however, deficient in weatherability and in resistance to pressure. When these seals are applied to joints, for example, stainless steel or other similar metal forming the joints is corroded and suffered to sustain pinholes. The seals made of urethane rubber, similarly to those of general-purpose rubber sheets, have an advantage that low pressure suffices for squeezing, a high contracting-resiling property is displayed, an electric insulating property is manifested, and deformation occurs only slightly even under high pressure. They, however, are relatively expensive and are deficient in resistance to acids. The seals made of polytetrafluoroethylene sheet have an advantage that high resistance to chemicals and resistance to cold are exhibited and an electric insulating property is manifested. They however, possess poor elasticity, require high pressure for squeezing, and are expensive. The seals made of polytetrafluoroethylene-covered asbestos enjoy an excellent contracting-resiling property in addition to the advantage of the seals of polytetrafluoroethylene sheet mentioned above. They, however, are producible only in limited size and shape. Further, they prove to be disadvantageous because they inevitably incorporate therein asbestos, a substance those use is now enthusiastically discouraged by reason of danger to safety. The seals made of carbon fiber knit, though superior to asbestos in resistance to chemicals and softness, are inferior to asbestos in resistance to heat and are expensive. The seals made of Alamides fiber knit exhibit tensile strength and elasticity substantially equal to those of the seals of asbestos. They, however, are expensive.
Though various constituent materials for seals have been developed to take the place of asbestos, they are not fully satisfactory from the standpoint of performance of cost. In the circumstances, a desired is expressed in the industry to develop new raw materials.
Seals such as O rings and packings are disposed in joints of pipes or containers laid in plants of the food industry, for example. These seals are required not only to abound with such properties as tensile strength, elasticity, sealing property, and resistance to chemicals but also to warrant high safety with respect to exudates.
Heretofore, as the seals such as O rings and packings are disposed in joints of pipes or containers laid in plants of the food industry, for example, those made of such raw materials as general-purpose rubbers, urethane rubber, or silicone rubber have been generally used. The seals made of such raw materials as general-purpose rubbers or urethane rubber are not fully satisfactory, as described above, in such properties as tensile strength, elasticity, sealing property, and resistance to chemicals and, more often than not, they prove to be unsatisfactory in terms of the safety with respect to exudates. The seals made of silicone rubber, in contrast, exhibit strength, elasticity, sealing property, resistance to chemicals, and resistance to heat sufficiently and prove to be highly satisfactory in terms of the safety with respect to exudates in the application under discussion. Unfortunately, they are expensive. Thus, the desirability of developing seals which possess equal or more desirable properties and which are not expensive has been finding growing recognition.